1. Field of the Invention
The present invention relates to an image forming apparatus, an image processing apparatus, an image processing method, and a recording medium.
2. Description of the Related Art
An electrophotographic color image forming apparatus such as a copying machine, an MFP, or a printer generally performs color correction processing based on density control using a predetermined test chart (patch images). In the color correction processing, for example, patch images for density detection are formed on a photosensitive member, an intermediate transfer member, or a transfer material carrier that carries the transfer material, and the densities of the patch images are detected using a density detection sensor. In addition, an image forming condition such as a charging bias, a developing bias, or an exposure amount in the image forming process is corrected in accordance with the detected density, thereby performing color correction.
In such color correction processing, the color correction is generally done for each color by forming patch images of the respective colors. However, it may be impossible to appropriately cope with an image density variation caused when developing materials (toners) of a plurality of colors are superposed. For example, when a toner image is formed by superposing a plurality of color toners, a toner layer thicker than that of a toner image of a single color toner is formed. This may lead to a transfer error that changes the appropriate transfer voltage and degrades the toner image transfer performance.
For example, assume that a magenta (M) toner layer is formed on the surface of the intermediate transfer member, and a cyan (C) toner layer is superposed on it to form a multicolor toner image. When transferring the toner image from the intermediate transfer member to a recording material, the C toner layer is directly transferred onto the recording material, and the M toner layer is transferred on it. At this time, a transfer error may occur so that only part of the M toner layer that is farther from the recording material surface than the C toner layer is transferred to the recording material, and the other part remains on the intermediate transfer member due to shortage of the transfer voltage or the like. If such a transfer error occurs, the color obtained by mixing the plurality of toners superposed and transferred to the recording material becomes too close to the color (C, in this case) of the toner layer at a shorter distance from the sheet surface, resulting in degradation of multicolor reproducibility of the formed image.
To cope with the above-described degradation of multicolor reproducibility caused by the transfer error, Japanese Patent Laid-Open No. 2006-259142 proposes a method of printing a test chart including a patch image of a multicolor expressed by overlaying color toners and performing color correction based on the colorimetric result of the printed test chart. This method allows to detect a process failure that causes degradation of color reproducibility of a high-density multicolor.
An image defect such as the above-described degradation of color reproducibility or color unevenness caused when forming a patch image can occur not only in the transfer processing but also in fixing processing of fixing the transferred toner image onto a sheet. In general, the surface of plain paper is uneven with a step of several μm. When fixing a toner image transferred to such plain paper, the thickness of the fixed toner layer may change between regions of the plain paper surface depending on the fixing temperature. More specifically, when the fixing temperature or the pressure in the fixing processing is high, the molten toner is pressed from the convex portions into the concave portions of the plain paper surface, resulting in an uneven toner layer thickness. The color is developed to high saturation at the concave portions where the toner layer is thick and to low saturation at the convex portions where the toner layer is thin. The color of the formed patch image is developed to low saturation especially because of the thin toner layer portions. In addition, the saturation unevenness occurs as color unevenness for each small region of the patch image in accordance with the unevenness of the sheet.
To reduce the above-described color unevenness caused upon transfer or fixing by color correction processing so as to suppress an image defect, it is necessary to quantitatively evaluate the image defect. Methods of quantitatively evaluating an image defect are proposed in, for example, Japanese Patent Laid-Open Nos. 2006-259142 and 2006-174182. In the method proposed in Japanese Patent Laid-Open No. 2006-259142, a transfer error or a fixing error is detected based on the difference between a predetermined target value and a colorimetric value obtained by performing colorimetry of a multicolor patch image, as described above, and the process conditions of the image forming apparatus are adjusted based on the detection result. In the method proposed in Japanese Patent Laid-Open No. 2006-174182, color unevenness is quantitatively evaluated based on the maximum value of the color difference concerning a color value obtained by reading an evaluation pattern output from the image forming apparatus as image data.
However, as an important factor that can affect subjective evaluation of an image including color unevenness, the spatial frequency characteristic of color unevenness is known. For this reason, to objectively and accurately evaluate color unevenness, the spatial frequency of the color unevenness needs to be taken into consideration. In Japanese Patent Laid-Open No. 2006-174182, however, since the image quality is quantified without considering the spatial frequency of color unevenness, it is difficult to objectively and accurately evaluate an image defect caused by color unevenness.
Furthermore, in a patch image formed for color correction, the amount of applied toner may change depending on the performance difference generated at the time of manufacture of the image forming apparatus main body or its consumables, or the life, use environment, printing environment, or printing condition of the image forming apparatus main body or its consumables. For this reason, even if no color unevenness is caused by transfer or fixing, the color of the patch image is not always constant. Hence, when detecting an image defect based on the difference between the colorimetric value and the target value of the patch image, as in Japanese Patent Laid-Open No. 2006-259142, it is difficult to determine what leads to the detection result, color unevenness caused by transfer or fixing or the variation in the amount of applied toner caused by the change of the printing environment or the like.